2001
Flight altitudes of coastal birds
203
FLIGHT ALTITUDES OF COASTAL BIRDS IN
RELATION TO WIND DIRECTION AND SPEED
THORSTEN KRÜGER1,3 & STEFAN GARTHE2,3
Krüger, T. & Garthe S. (2001): Flight altitudes of coastal birds in relation to wind direction and
speed. Atlantic Seabirds 3(4): 203-216. During systematic sea watches carried out between 1
September and 15 November 1999 on the East Friesian island of Wangerooge, observations were
recorded of the flight altitude of coastal birds in relation to wind direction and speed. In Redthroated Diver Gavia stellata, Common Eider Somateria mollissima and Common Scoter Melanitta
nigra the proportion of birds flying into the wind low over the water (0-1.5m) increased with wind
speed. On the other hand, in the same species, the number of low-flying birds decreased in inverse
proportion to the speed of a tail wind and the ratio of birds flying at greater altitudes increased
(1.5-12m and 12-25m respectively). Irrespective of wind speed, the proportion of individual birds
flying low into the wind was highest in Red-throated Diver, Shelduck Tadorna tadorna, Common
Eider and Common Scoter. This pattern is repeated at a higher level in Sandwich Tern Sterna
sandvicensis and Common/Arctic Terns S. hirundo/paradisaea. In contrast, in tail winds, the greatest
proportion of birds of these species invariably flew at the highest levels. Comparisons of flight
altitudes reveal that these species fly noticeably higher in tail winds. This behaviour can be
explained in terms of economy of effort on migration. The present study also reveals that diurnal
movement of the observed species takes place mainly at a low flight altitude (up to 25m,
occasionally up to 50m, rarely higher) above sea level. This demonstrates potentially adverse effects
on birds from construction of proposed offshore wind farms. The data indicate that, to be of any
value in the assessment of the potential disturbance of the wind farms to North Sea migrants, flight
altitude records must be viewed against the background of the meteorological situation as a whole.
1
Institut für Vogelforschung „Vogelwarte Helgoland“, An der Vogelwarte 21, D-26386
Wilhelmshaven, Germany; [email protected] 2Institut für Meereskunde, Düsternbrooker
Weg 20, D-24105 Kiel, Germany; [email protected], 3Present address: Forschungs- und
Technologiezentrum Westküste (FTZ) Universität Kiel, Hafentörn, 25761 Büsum Germany
INTRODUCTION
Little is known about the flight altitudes of coastal birds migrating through the
southern North Sea, and even less about their relation to meteorological
parameters. Statements about flight altitudes based on sea-watch observations
have generally been vague; scattered information about species migrating at a
particular height may be found in Berndt & Drenckhahn (1974), Camphuysen &
van Dijk (1983), Nehls & Zöllick (1990) and Temme (1995). Although radar
observations have improved our understanding of bird migration over the North
Sea substantially (Lack 1963; Jellmann 1979; Buurma 1987), they have not
been able to provide clear information on low-level migratory movement until
very recently; radar equipment used to date is mostly accurate only at heights
above (50) 100-300 m, and it is often not possible to identify birds to species
204
T. KRÜGER & S. GARTHE
Atlantic Seabirds 3(4)
level using radar (Williams & Williams 1990; Berthold 2000). For a long time
the intensity of low-level migratory movements over the sea generally were
thought - aside from the moult migration of Shelduck Tadorna tadorna- to be of
minor importance (cf. Jellmann 1979, 1987, 1989). Major improvements in
radar technology now enable exact measurements of flight altitudes (e.g.
Dirksen et al. 1996, 1998). However, such studies have been carried out so far
only at selected locations, usually to inform environmental issues such as wind
energy utilization.
The purpose of the study described here was to examine the flight
altitudes of the most common coastal birds and seabirds by field observations of
visible migration and to identify the influence of the meteorological parameters
wind direction and speed.
METHODS
Visible migration of coastal birds and seabirds was studied on 52 days between
1 September and 15 November 1999 on Wangerooge Island, Germany.
Wangerooge is located in the southern North Sea (German Bight; 53°47'N
07°54'E) and constitutes the easternmost of the East Frisian islands.
Observations followed common sea watching methods (Camphuysen & van
Dijk 1983) using a telescope (Swarovski, AT 30x80 HD). We excluded all
common gull species from the observations (Larus argentatus, L. fuscus, L.
marinus, L. canus, L. ridibundus) because it was impossible to distinguish
between true migration and frequent local movements between resting and
feeding sites. The method of sea watching has been proved suitable for
describing and analysing migratory behaviour (e.g. phenology, influence of
wind) of birds along the coast and at sea (e.g. Camphuysen & van Dijk 1983,
Platteeuw et al. 1994). It is not the purpose of this study to assess all birds flying
during day and night, which could be done much more comprehensively by
radar studies.
In order to study whether wind direction and wind speed influence
directly the flight altitude of migrating birds, all individuals passing
Wangerooge were allocated to one of several pre-arranged flight levels. Because
the estimation of flight altitude is difficult in general and because "observers
nearly always overestimate the height of migrating birds" (Thienemann 1931,
see also Gätke 1900, Lucanus 1923), a simple but accurate schema had to be
devised (Fig. 1):
•
Birds flying low over the water were allocated to the lowest level (low). The upper
limit of this level was about 1.5 m. In light winds and relatively calm seas it was
easy to identify this level. In these conditions, the birds often flew so low over the
2001
Flight altitudes of coastal birds
205
25 m
high
12 m
medium
Wangerooge
1,5 m
low
sea level
Figure 1. Allocation of flight altitude above the sea surface to various predetermined levels.
Figuur 1. Verdeling van vlieghoogte in hoogteklassen ten opzichte van het
zeeoppervlak.
•
•
•
surface that the reflection of their wingtips in the water was clearly visible at
considerable distances. In rougher or stormy seas, high waves frequently forced the
birds to leave their low trajectories for at least part of the time. To be sure of a
correct flight altitude allocation, the observers were obliged to follow the flight
paths over longer distances in order to distinguish between a medium flight altitude
and a tendency towards a low trajectory.
The upper limit of the next higher level (medium) was at eye-level for the observers
and thus encompassed the space from about 1.5 m to 12 m a.s.l. This categorised all
those birds that were not clearly flying "low" and yet not flying above eye-level (=
side view). Birds in flight above eye-level, and thus viewed at an angle from below,
were allocated to a third level (high), whose upper limit was assigned to about 25 m.
A further category (very high) was established for birds flying even higher, at flight
altitudes > 25 to 100 m.
The levels low, medium and high lay all within the telescope’s field of vision. The
observations were regulary interrupted by searching the sky for very high-flying
birds with a binocular (Zeiss 10x40). As a rule, birds passing >3 km away were
excluded from flight altitude assessments as these could not be estimated accurately.
206
T. KRÜGER & S. GARTHE
Atlantic Seabirds 3(4)
Table 1. The correlation (r) between the percentage of birds passing Wangerooge in
autumn 1999 and wind speed, grouped according to wind direction and flight
altitude. Correlation coefficient r is in italics where p< 0.05 (cf. Fig. 2).
Tabel 1. Correlatie (r) tussen het aandeel vogels dat Wangerooge in het najaar van
1999 passeerde en de windsnelheid, gegroepeerd naar windrichting en
vlieghoogte. Correlatiecoëfficiënt r is cursief indien p < 0.05 (cf. Figuur 2).
Red-throated Diver
Common Eider
Common Scoter
Headwind
Low
0.295
0.550
0.223
Tail wind
Low
-0.720
-0.770
-0.668
To minimise distortion of the results through other meteorological
factors, rainy, hazy and foggy days during the period of the investigation were
excluded from the evaluation (cf. Bruderer 1971). In order to correlate the
observed migration with the wind, measurements of wind speed and wind
direction were taken from a meteorological station on Minsener Oog (8 km E of
Wangerooge;
Wasserund
Schiffahrtsamt
Wilhelmshaven,
Abt.
Gewässerkunde). During the study, the main migration direction was west.
Consequently, winds from SW-NW are defined as headwinds, those from SENE as tail winds.
RESULTS
Flight altitudes in relation to wind speed and direction
Red-throated Diver Gavia stellata During headwinds, most Red-throated
Divers migrated low over the water (range: 60-100%; Fig. 2). The proportion of
low-flying birds increased with increasing wind speed, and was up to 100%
beyond speeds of 10.8 m s-1. High-flying individuals were seldom recorded in
headwinds and even then the breezes were only light.
During light tail winds, the greatest percentage of Red-throated Divers
were still low-flyers, even though this decreased significantly with increasing
wind speed (Table 1, Fig. 2). No low flying Red-throated Divers were recorded
in tail winds of over 15.4 m s-1. Thus, the proportion of birds flying at medium
height became greater with increasing wind speed. High-flying Red-throated
Divers – rarely observed in headwinds – made up as much as 90% of the
individuals in tail winds. Their proportion increased in general with wind speed.
2001
Flight altitudes of coastal birds
Tail wind
100
80
60
low
40
20
0
0
5
10
15
20
Number of birds [%]
Number of birds [%]
Headwind
100
80
60
low
40
20
0
25
0
5
Windspeed [m s-1]
60
low
40
20
0
15
Number of birds [%]
Number of birds [%]
80
10
60
low
40
20
0
0
10
30
Eider (n = 10,164)
80
60
low
40
20
0
15
20
Windspeed [m s-1]
Common Scoter (n = 4,030)
25
Number of birds [%]
Number of birds [%]
20
Windspeed [m s-1]
100
10
25
80
20
Eider (n = 4,241)
5
20
100
Windspeed [m s-1]
0
15
Red-throated Diver (n = 111)
100
5
10
Windspeed [m s-1]
Red-throated Diver (n = 147)
0
207
100
80
60
low
40
20
0
0
5
10
15
20
25
Windspeed [m s-1]
Common Scoter (n = 2,724)
Figure 2. Flight altitude of westward migrating coastal birds in headwinds (NWSW) and in tail winds (NE-SE). Lines = linear regression per flight level;
correlation coefficient r: see Table 1.
Figuur 2. Vlieghoogte van westwaarts vliegende kustvogels bij tegenwind (NW-ZW)
en bij meewind (NO-ZO). Lineaire regressielijn per vlieghoogte; zie tabel 1
voor waarden van correlatiecoëfficiënt r.
208
T. KRÜGER & S. GARTHE
Atlantic Seabirds 3(4)
Common Eider Somateria mollissima The low flight path was the most
frequently chosen by this species (46-72%; Fig. 2), increasingly so with stronger
headwinds. The medium level was used most frequently at wind speeds of 4.1 m
s-1 (42%), this proportion decreasing with stronger winds. The percentage of
high-flying Common Eider dropped from 34% at 4.1 m s-1 to 0% at 13.4 m s-1
and more.
In light tail winds Common Eiders mainly flew low over the surface of
the water (2 m s-1: 68%; Fig. 2). The low level was used less with increasing
wind speed, and was not used at all in winds in excess of 13.3 m s-1. Instead, the
proportion of high-flying Common Eider increased, reaching 86% at wind
speeds of 13.3 m s-1. The proportion of Common Eiders flying at medium height
tended to remain constant with increasing wind speeds.
Common Scoter Melanitta nigra The percentage of Common Scoters flying
low into the wind increased with increasing wind speed, reaching values of over
90% at speeds of 10.3 m s-1 and above (Fig. 2). Correspondingly, the stronger
the wind, the smaller the number of Common Scoters migrating at medium
level. Only 1-9% of the Common Scoters observed flew high into the wind. In
tail winds of increasing strength, Common Scoters became increasingly
reluctant to fly low over the surface of the water (Fig. 2). Instead, the percentage
of birds migrating at medium height rose and eventually predominated. 5-39%
of the Common Scoters passed at the high level but increasing wind speeds did
not result in a clear trend.
Flight altitudes in relation to wind direction Considered without reference to
prevailing wind speeds, westward moving Red-throated Divers used most
frequently the low level in winds from the NW-SW sector (headwind; 83%; Fig.
3). They rarely flew higher (medium: 15%, high: 2%). In winds from the NE-SE
sector (tail winds), the two (or three) higher flight levels were used more often:
low = 34%, medium = 27%, high = 38%, very high = 1%. The pattern of flight
altitude allocation in headwind conditions differed significantly from that in tail
winds (Table 2).
When flying against the wind, approximately 75% of Shelduck passed
low over the water and 23% selected the next higher level (Fig. 3). In tail winds,
a preference for the high and very high levels was detected (51% and 3%
respectively); the percentage of low-flying birds was 15%. In headwinds, the
majority (58%) of Common Eider migrated low over the water, but large
numbers also chose the medium level (34%; Fig. 3). In tail winds, the greatest
number of Common Eiders flew past at medium height (38%), but almost as
many (31%) – certainly much more than in Red-throated Divers and Common
Scoters – migrated at high level. Westward migrating Common Scoters mostly
2001
Flight altitudes of coastal birds
209
Table 2. The numbers of coastal migrants passing Wangerooge in autumn 1999,
grouped according to wind direction and flight altitude. χ² = significance of
the 2*3 contingency tables for differences between headwinds and tail winds.
p < 0.001.
Tabel 2. Aantal trekvogels dat Wangeroog najaar 1999 passeerde, gegroepeerd naar
windrichting en vlieghoogte. p < 0.001.
Headwind
low medium high
Red-throated Diver 113
21
2
Shelduck
218
70
17
Common Eider
2442
1428
371
Common Scoter
3286
711
33
Sandwich Tern
179
560
40
Comm/Arctic Tern
5
117
26
Tailwind
low medium
38
30
107
221
2998 3850
1022 1389
0
63
0
33
high
43
380
3316
308
117
90
χ²
74.4
349.9
1305.9
1429.8
392.1
85.8
flew low over the water during headwinds, whereas the medium level was used
most often during tail winds (51%; Fig. 3).
Most Sandwich Terns Sterna sandvicensis flying into the wind clearly
preferred the medium level (72%) and a further 23% selected the low level (Fig.
3). In tail winds the high level was most frequently used (65%); 35% flew at
medium height, and there were no records of low-flying migrating Sandwich
Terns. 79% of Common/Arctic Terns S. hirundo/paradisaea migrated in
headwinds at medium levels and another 18% flew high (Fig. 3). Headwinds
saw the bulk (73%) of the birds in the high flight path, whereas only 23% were
to be found at medium height.
Even within a single observation day it became evident that flight
altitude was influenced by the direction of the wind. During the autumn
migration period, Common Eiders off Wangerooge regularly moved in both
easterly and westerly directions (see also Camphuysen & van Dijk 1983,
Platteeuw et al. 1994). On days during which migration took place in both
directions, one of the routes – eastward or westward – had headwinds. On nine
headwind days (only days with more than 1,000 individuals were considered),
Common Eiders migrated for the most part low in a westerly direction.
Conversely, birds moving east on those days mostly had a medium or high flight
altitude (Fig. 4). In same-day comparisons, they always moved one or two levels
higher up. On nine tail wind days, however, the mass of the birds migrated
medium or high levels in a westerly direction. The birds moving east for the
most part flew low over the water (Fig. 4).
210
T. KRÜGER & S. GARTHE
very high
high
headwind
medium
tail wind
Flight altitude
very high
Atlantic Seabirds 3(4)
low
high
headwind
medium
tail wind
low
-100
-50
0
50
100
-100
Number of birds [%]
Red-throated Diver (n = 247)
50
100
very high
high
headwind
medium
tail wind
Flight altitude
Flight altitude
0
Eider (n = 14,405)
very high
high
headwind
medium
tail wind
low
low
-100
-50
0
50
-100
100
Common Scoter (n = 6,754)
0
50
100
Shelduck (n = 1,013)
very high
very high
high
high
headwind
medium
tail wind
headwind
tail wind
medium
low
low
-100
-50
Number of birds [%]
Number of birds [%]
Flight altitude
-50
Number of birds [%]
-50
0
50
100
Number of birds [%]
Sandwich Tern (n = 959)
-100
-50
0
50
100
Number of birds [%]
Common/Arctic Tern (n = 271)
Figure 3. Relative frequency of westward moving coastal birds, grouped according to
flight altitude and wind direction.
Figuur 3. Relatieve verdeling van westwaarts vliegende kustvogels, gegroepeerd naar
vlieghoogte en windrichting.
2001
Flight altitudes of coastal birds
211
11.10.
12.10.
14.10.
27.10.
31.10.
02.11.
03.11.
04.11.
15.11.
25
12
1,5
1,5
12
25
Flight altitudes, upper limits [m]
15.10.
16.10.
18.10.
19.10.
24.10.
08.11.
10.11.
11.11.
14.11.
25
12
1,5
1,5
12
25
Flight altitudes, upper limits [m]
Figure 4 . Common Eider flight altitudes on days with NW-SW winds off Wangerooge
(upper) and Common Eider flight altitudes on days with NE-SE winds off
Wangerooge (lower), autumn 1999. The bars show in each case the most
frequent level; westerly migrants to the left, easterly to the right of the central
axis.
Figuur 4. Vlieghoogtes van Eiders op dagen met NW-ZW-winden langs Wangerooge
(boven) en vlieghoogtes op dagen met NO-ZO-winden (onder), najaar 1999.
De staven geven voor iedere dag de meest frequente vlieghoogte weer;
westwaarts vliegende vogels links, oostwaarts vliegende vogels rechts van de
y-as.
212
T. KRÜGER & S. GARTHE
Atlantic Seabirds 3(4)
Black-throated Divers Parelduikers (Frits-Jan Maas)
DISCUSSION
According to Bruderer (1971), migration altitude is influenced – always within
the limits set by the aerodynamic and physiological characteristics of the species
– by secondary (external) factors, principally meteorological factors such as
wind, fog, cloud conditions and precipitation, and changes in overall weather
conditions. Among these, wind is of greatest importance (Alerstam 1979a). The
horizontal speed of wind over the sea increases with height; directly above the
water surface there is a zone of lower wind speed caused by the breaking effect
of the water. Jameson (1960) describes a case where the wind speed just 15 m
above sea level was at force 8 Bft, twice that immediately above the water
surface. Wind reaches its full force only above 500 m (Alerstam 1979b;
Nachtigall 1987; Stein & Schultz 1995).
Assuming that the radar ornithological results apply, the observations
of the present study confirm those of numerous investigations showing that the
height of migration in tail wind conditions is greater than that in headwinds, and
that where wind direction is opposed to the migration direction, in general lower
2001
Flight altitudes of coastal birds
213
flight altitudes with attendant lower wind speeds are selected (Bruderer 1971;
Karlsson 1976; Kumari 1983; Bruderer 1997; Bruderer & Liechti 1998).
The reason for the migratory behaviour observed off Wangerooge is
probably that birds need to save time and energy when on migration (cf.
Berthold 2000). In general, active flying costs birds much energy per unit of
time. In order to minimise these costs, some bird species have developed flight
techniques that enable them to cover long distances with less energy expenditure
(e.g. gliding and soaring, formation flying; see reviews by Rüppel 1980;
Norberg 1996). Still other species employ alternating flight techniques, such as
the shearwaters, which flap-glide, alternately beating their wings and sheering
through the wave-troughs. These means of saving energy are not available to all
species. Some species have unfavourable wing area to body mass ratios (wing
loading) and find it impossible to glide or soar long distances over water; such
species are obliged to propel themselves forward by continuous wing-beating
(swans, geese, ducks, auks etc., Rüppel 1980; Pennycuick 1987 a, b). The
species under investigation in this paper belong mostly to the latter group. The
most efficient way for these birds to minimise energy costs of migrating upwind in terms of optimal migration (Alerstam & Lindström 1990) is to fly low –
the stronger the headwind, the lower the flight level. In tail winds, they migrate
more efficiently at greater heights (see also Gatter 2001). The present study
suggests, that this pattern is also valid for migration across the North Sea at
low(er) altitudes.
Behaviour like this makes it possible for birds in some cases almost to
double their flight speed and to halve their energy costs (Liechti & Bruderer
1998; Liechti & Schaller 1999; Liechti et al. 2000). These relationships have
already been found by Bellrose (1967) who summarised the findings of his radar
studies: "[...] demonstrate that birds have a phenomenal understanding of winds.
They select [...] altitudes having favourable directional winds and favourable
wind speeds." The data on flight altitudes presented in this paper are
predominantly based on migration flights (Krüger 2001). It is, however, not
possible to quantify the extent to which they have been influenced by
compensating movements or have been caused by disturbances. Those shorter
flights could be performed in a different manner (e.g. lower flight altitudes) and
under different energy saving strategies as real migration flights.
An interesting result of this study is that the visible, diurnal movements
of the species investigated take place mainly at low heights above the water
surface (up to 25 m, occasionally up to 50 m, rarely higher). This accords with
other observations of bird migration in coastal areas. Thus 75% of migrating
Red-throated Divers in the Fehmarn Belt (Baltic Sea) in the winter of 1956/57
were observed between 6 and 15 m (mean flight altitude: 10.5 m, maximum: 45
m). In other years also, flight altitudes of 60-100 m were registered during tail
214
T. KRÜGER & S. GARTHE
Atlantic Seabirds 3(4)
wind conditions (Drenckhahn et al. 1974). Temme (1974) found that the
majority of Common Eiders off Norderney (southern North Sea) migrated
between 1 and 3 m over the water surface, in rare occasions up to 20-25 (45) m.
Common Scoters off Rügen (Baltic Sea) were recorded migrating usually at
altitudes of 1-2 m; in strong tail winds they reached 10-100 m (Nehls & Zöllick
1990).
The findings of this study are not only interesting in terms of coastal
bird ecology but also with respect to current environmental issues in the
southern North Sea. Extensive plans to construct windmill farms in offshore
areas of the German Bight and other areas may create a substantial risk for
migrating birds at sea. In order to study the potential risk with regard to collision
(which causes mortality) and flight route disturbance, comprehensive
environmental impact assessments need to be carried out (e.g. Garthe 2000).
Such studies should consider flight altitudes of birds also in relation to the
prevailing meteorological situations.
ACKNOWLEDGEMENTS
K. Wilson helped with the English language. P.H. Becker, C.J. Camphuysen, W. Gatter and an
anonymous referee gave valuable comments on the manuscript. The Mellumrat e.V. made it possible
to stay on Wangerooge. The Wasser- und Schiffahrtsamt Wilhelmshaven provided meteorological
data. M. Feuersenger, R. Lottmann, J. Dierschke and M. Heckroth helped with the fieldwork and
discussed the method.
VLIEGHOOGTES VAN KUSTVOGELS IN RELATIE
TOT WINDRICHTING EN WINDSNELHEID
Tijdens systematische zeetrektellingen tussen 1 september en 15 november 1999 op het Oost-Friese
eiland Wangerooge, werden waarnemingen verzameld over de vlieghoogte van kustvogels in relatie
tot windrichting en -snelheid. Bij Roodkeelduiker Gavia stellata, Eider Somateria mollisima en
Zwarte Zee-eend Melanitta nigra nam het aandeel laag (0-1.5 m)tegen de wind in vliegende vogels
toe met toenemende windsnelheid. Het aandeel laag vliegende vogels nam met meewind omgekeerd
evenredig af met de windsnelheid, terwijl het aandeel hoger vliegende vogels (1.5-25 m) toenam.
Het aandeel individuen dat tegen de wind invloog was, ongeacht de windsnelheid, het hoogst bij
Roodkeelduiker, Bergeend Tadorna tadorna, Eider en Zwarte Zee-eend. Grote Stern Sterna
sandvicensis en Noordse Dief S.hirundo/paradisaea vertonen een zelfde beeld, zij het op grotere
hoogte. Met meewind vloog het merendeel van deze soorten in de bovenste hoogteklassen.
Vergelijking van de vlieghoogtes laat zoen dat alle soorten met meewind aanmerkelijk hoger
vliegen. Dit gedrag kan verklaard worden uit energetisch oogpunt. Deze studie laat tevens zien dat
dagtrek van de waargenomen soorten met name op lage hoogtes (tot 25 m, soms tot 50 m, zelden
hoger) boven zee plaatsvindt. De gegevens wijzen op potentiële, schadelijke effecten op vogels van
de bouw van voorgestelde offshore windmolenparken. Om van enig nut te zijn bij risico-analyse
van potentiële verstorende effecten van windmolenparken op door de Noordzee trekkende vogels
moeten waarnemingen van vlieghoogtes tegen de achtergrond van de gehele metereologische
situatie beschouwd worden.
2001
Flight altitudes of coastal birds
215
REFERENCES
Alerstam T. 1979 a. Wind as selective agent in bird migration. Ornis Scand. 10: 76-93.
Alerstam T. 1979 b. Optimal use of wind by migrating birds: combined drift and overcompensation.
J. Theor. Biol. 79: 341-353.
Bellrose F.C. 1967. Radar in orientaion research. Proc. 14th Int. Ornithol. Congr., Oxford 1966: 281309.
Berndt R.K. & Drenckhahn D. 1974 (eds). Vogelwelt Schleswig-Holsteins, Vol. 1. Neumünster.
Berthold P. 2000. Vogelzug. Eine aktuelle Gesamtübersicht, Darmstadt.
Bruderer B. 1971. Radarbeobachtungen über den Frühlingszug im Schweizerischen Mittelland (Ein
Beitrag zum Problem der Witterungsabhängigkeit des Vogelzugs). Ornithol. Beob. 68: 89-158.
Bruderer B. 1997. The study of bird migration by radar. Part 2: Major achievements. Naturwiss. 84:
45-54.
Bruderer B. & Liechti F.1998. Intensität, Höhe und Richtung von Tag- und Nachtzug im Herbst über
Südwestdeutschland. Ornithol. Beob. 95: 13-128.
Buurma L.S. 1987. Patronen van hoge vogeltrek boven het Noordzeegebied in oktober. Limosa 60:
63-74.
Camphuysen C.J. & van Dijk J. 1983. Zee- en kustvogels langs de Nederlandse kust, 1974- 79.
Limosa 56: 83-211.
Dirksen S., Spaans A.L. & van der Winden J. 1996. Nachtelijke trek en vlieghoogtes van steltlopers
in het voorjaar over de noordelijke havendam van IJmuiden Sula 10: 129- 142.
Dirksen S., Spaans A.L., van der Winden J. & van den Bergh L.M.J. 1998. Nachtelijke
vliegpatronen en vlieghoogtes van duikeenden in het IJsselmeergebied. Limosa 71: 57- 68.
Drenckhahn D., Gloe P. & Heldt R. 1974. Sterntaucher Gavia stellata. In: Berndt R.K. &
Drenckhahn D. (eds). Vogelwelt Schleswig-Holsteins, Vol. 1. Neumünster.
Gätke F. 1900. Die Vogelwarte Helgoland, Braunschweig.
Garthe S. 2000. Mögliche Auswirkungen von Offshore-Windenergieanlagen auf See- und
Wasservögel der deutschen Nord- und Ostsee. In: Merck T. & von Nordheim H. (eds.):
Technische Eingriffe in marine Lebensräume. Workshop of the "Bundesamt für Naturschutz,
Internationale Naturschutzakademie Insel Vilm", 27 - 29 October 1999. BfN-Skripten 29: 113119.
Gatter W. 2001. Vogelzug und Vogelbestände in Mitteleuropa. 30 Jahre Beobachtung des Tagzugs
am Randecker Maar. Wiesbaden.
Jameson W. 1960. Flight of the Albatross. Natural History 69: 62-69.
Jellmann J. 1979. Flughöhen ziehender Vögel in Nordwestdeutschland nach Radarmessungen.
Vogelwarte 30: 118-134.
Jellmann J. 1987. Radarbeobachtungen zum nächtlichen Mauserzug der Brandgans (Tadorna
tadorna) an der Nordseeküste. Seevögel 8: 63-64.
Jellmann J. 1989. Radarmessungen zur Höhe des nächtlichen Vogelzuges über Nordwestdeutschland
im Frühjahr und im Hochsommer. Vogelwarte 35: 59-63.
Karlsson J. 1976. Radar measurements of migration altitudes of Eiders (Somateria mollissima) over
southernmost Sweden. Fauna Flora, Uppsala 71: 152-157.
Kumari E. 1983. Characteristics of seaduck movements in the Baltic. Ornis Fennica Suppl. 3: 39-40.
Krüger T. 2001. Untersuchungen zum Zugverhalten ausgewählter See- und Küstenvögel in der
südlichen Nordsee. Diplomarbeit. Universität Oldenburg.
Lack D. 1963. Migration across the southern North Sea studied by radar. Part 4. Autumn. Ibis 105:
1-54.
Liechti F., Klaassen M. & Bruderer B. 2000. Predicting migratory flight altitudes by physiological
optimal migration models. Auk 117: 205-214
Liechti F. & Schaller E. 1999. The use of low-level jets by migrating birds. Naturwissenschaften 86:
549-551.
216
T. KRÜGER & S. GARTHE
Atlantic Seabirds 3(4)
Lucanus F. v. 1923. Die Rätsel des Vogelzugs, Langensalza.
Nachtigall W. 1987. Vogelflug und Vogelzug, Hamburg.
Nehls H.W. & Zöllick H. 1990. The moult migration of the Common Scoter (Melanitta nigra) off
the coast of the GDR. Baltic Birds 5: 36-46.
Norberg U. 1996. Energetics of Flight. In: Carey C. (ed). Energetics and nutritional ecology. New
York.
Pennycuick C.J. 1987a. Flight of seabirds. In: Croxall J.P. (ed). Seabirds: feeding, ecology and role
in marine ecosystems. Cambridge, New York, Melbourne.
Pennycuick C.J. 1987b. Flight of auks (Alcidae) and other northern seabirds compared with southern
Procellariiformes: Ornithodolite observations. J. Exp. Biol. 128: 335- 347.
Piersma T. & van de Sant S. 1992. Pattern and predictability of potential wind assistance for waders
and geese migrating from West Africa and the Wadden Sea to Siberia. Ornis Svecica 2: 55-66.
Platteeuw M., van der Ham N.F. & den Ouden J.E. 1994. Zeetrektellingen in Nederland in de jaren
tachtig. Sula 8, Spec. Issue: 1-203.
Rüppell G. 1980. Vogelflug, Reinbek.
Stein W. & Schultz H. 1995. Wetterkunde für Segler und Motorbootfahrer, Bielefeld.
Temme M. 1974. Zugbewegungen der Eiderente (Somateria mollissima) vor der Insel Norderney
unter besonderer Berücksichtigung der Wetterverhältnisse. Vogelwarte 27: 252-263.
Temme M. 1995. Die Vögel der Insel Norderney, Cuxhaven.
Thienemann J. 1931. Vom Vogelzuge in Rossitten, Melsungen.
Williams T.C. & Williams J.M. 1990. The orientation of transoceanic migrants. In: Gwinner E. (ed.)
Bird Migration: The Physiology and Ecophysiology, Berlin.